The field of the invention relates to telephone systems and more particularly to the processing of call control signals at switching offices of telephone systems.
The use of R2 multifrequency compelled signalling (R2 MFC) in telephone systems is known. R2 MFC is an in-band signalling protocol that uses combinations of frequencies to convey information. While it is similar to dual tone multifrequency (DTMF) in that combinations of frequencies are used to convey information, R2 MFC provides the additional functionality of being adapted for use in inter-switch signalling.
More specifically, DTMF is typically used to convey dialed number information from a subscriber telephone to a local central office (CO). As such, DTMF is typically limited to the conveyance of digits 0 to 9, # and *. If a digit is not received by the CO, or is received incorrectly, the result is a wrong number or the familiar audio message xe2x80x9cthis call cannot be completed as dialed, please try your call againxe2x80x9d.
In contrast, R2 MFC is used between a CO and a local switching center. Further, R2 MFC, by necessity, is an interactive protocol.
For example, in placing a call from a CO to a switching center, the CO first seizes a trunk connection and forwards a make call request in the form of a R2 MFC frequency combination (e.g., F1+F2) over the seized connection. The CO then waits for a response. If the CO does not receive a response within a predetermined period, the request is repeated.
The switching center upon receiving the make call request (e.g., by detecting the R2 MFC combination of F1+F2) responds by acknowledging the make call request (e.g., by responding with a handshaking signal). The acknowledgement is returned to the CO and may be recognized at the CO by detection of a second frequency combination (e.g., F3+F4).
The CO may respond to the acknowledgement by forwarding a set of dialed digits. Again the CO may wait for a response and, if failing to get an acknowledgement, may again forward the set of dialed digits.
The switch upon receiving a set of dialed digits may respond with an acknowledgement. The acknowledgement may be in the form of another tone combination (e.g., F5+F6).
After receipt of the acknowledgment for the dialed digits, the CO again enters a wait state for either connection of the caller to the called number or for return of a busy signal. When a call is connected, the switch may send a connect combination (e.g., F7+F8). A busy signal may be indicated with a busy signal combination (e.g., F9+F10).
When the called party hangs up at the end of the call, the switch may return a called party termination signal combination (e.g., F11+F12). Similarly, where the caller hangs up first, the CO may send a calling party termination combination signal (e.g., F13+F14).
While the R2 MFC format works relatively well, it is also relatively slow. Further, other protocols have been developed (e.g., ISDN) for intra-switch communication which are regarded as much more efficient and considerably more flexible.
One difficulty with implementing newer switching protocols, however, lies in the expense of converting whole systems to the faster protocols. Further, converting less than all of the system provides inherent communication problems where one part of a system operates on one protocol and another part operates under another.
One solution to the problem of different parts of a system using different intra-switch communication protocols has been solved by the introduction of the Acculab Groomer. The Acculab Groomer is a translator box which is interposed in trunk lines between switches. On one side of the Acculab Groomer, communications are accomplished using R2 MFC. On the other side, the U.K. version of ISDN (i.e., DPNSS) is used.
While the Acculab Groomer is effective, it is also expensive to use and is inflexible in adapting to changing trunk line requirements or to a mix of trunk-line protocols. Consequently, a need exists for a means of adapting trunk interfaces to other formats without the necessity of translator boxes.
A method and apparatus are provided for processing signals received on a R2 multifrequency compelled signalling subscriber channel within a telephone switch. The method includes the steps of disposing a digital signal processor within the telephone switch between an inbound path of the R2 multifrequency compelled signal subscriber channel and a controlling processor of the telephone switch and detecting an R2 multifrequency compelled signalling control transmission by the digital signal processor on the inbound path. The method further includes the steps of responding to the control transmission by the digital signal processor transmitting an acknowledgement on an outbound path of the subscriber channel and transferring a symbolic representation of the control transmission to the controlling processor of the switch.